Natural oxygenation of the blood occurs in the body by the combination of the interaction of the lungs inhaling oxygen at a partial pressure higher than the partial pressure of the oxygen in the blood associated with the lungs, such that molecules of oxygen are transferred across the lung wall, made up of millions of alveoli, and into the blood stream, and with molecules of carbon dioxide passing the opposite direction for discharge from the lungs by exhaling.
Many mechanical, or extra-corporeal blood oxygenators have been tried through the years, as shown in U.S. Pat. Nos. 2,827,901; 3,046,903 and 3,325,641. These and other like devices are unsatisfactory because of damage to the blood. Whether forcing blood under high pressure through a nozzle to be torn into thousands of droplets, or forcing blood through capillary-liketubes about which oxygen is entrained, the shear forces exerted on the red blood cells cause excessive and dangerous hemolysis. Other devices attempt to utilize a large flat surface area in order to obtain adequate gas or fluid transfer through the membrane; however, an increase of the guage pressure of the gas sufficient to obtain satisfactory transfer increases the danger of rupture of the membrane such that the gas, oxygen, is then transferred directly into the bloodstream, a deadly result.
Extracorporeal blood oxygenators may be classified into three major categories: bubble oxygenators, disc oxygenators, and membrane oxygenators. The first two types oxygenate the blood by exposing it directly to the gas phase. This is an efficient method of oxygenation, but due to the direct blood/gas interface, the oxygenators (the bubble more so than the disc) damage the formed elements of the flood and various plasma proteins. In the membrane type oxygenator, a membrane, of which there are various types and designs, separates the blood from the gas phase. The blood damage is reduced significantly, but there are problems involving oxygenation due to the various flow parameters encountered, especially boundary layer phenomenon.
The first bubble type oxygenator was developed in 1950 by L. C. Clarke and his associates. The unit was relatively simple: oxygen was bubbled through a pool of venous blood, and the bubble/blood interface caused the oxygenating surface. This is the most widely used blood oxygenator for cardiopulmonary by-pass despite its high rate of blood damage, since most surgical procedures involving total by-pass rarely require more than an hour to complete.
The disc oxygenator was developed in 1948 by Bjork. This device consists of a series of flat discs which rotate on a central axis and which are surrounded by a chamber containing an ambient gas mixture which flows over and around the upper one-third of the discs; blood is exposed to the bottom two-thirds of the discs. As poorly oxygenated venous blood flows into the chamber, the rotation of the disc forms a thin film of blood on the upper one-third of each disc increasing the blood/gas interface. This process continues for the entire length of the oxygenator. The outlet blood was well oxygenated (approximately 98% saturated), and also, well hemolyzed because of the direct blood/gas interface and high shear forces.
The development of the first membrane blood oxygenator was in 1955 by G. H. A. Clowes, Jr. This oxygenator consists of a series of flat, ultra thin, plastic (ethyl cellulose) membranes which separate the blood phase from the gas phase. The oxygenating capacity of the oxygenator is dependent upon the diffusion of oxygen and carbond dioxide through the membrane. This initial development was followed by various models all operating in the same general manner.
The immediate advantage of the membrane oxygenator was the elimination of the blood/gas interface. Trauma to the formed elements and plasma proteins of the blood was significantly reduced, the oxygenation process being the major problem. A large surface area was needed to oxygenate the blood sufficiently. Consequently, this required many stacks of large, flat membranes that rendered the unit cumbersome. For example, the first oxygenator of Clowes et al was almost 1 foot by 4 feet with a stack of membrane units nearly a foot thick! In addition to the bulkiness of the oxygenator, the membranes were thin and flat, and, therefore, extremely fragile. This became especially hazardous with the development of the fragile silicone rubber membranes. Ruptures were frequent which allowed the formation of gas bubbles in the blood. The large size of most of these units meant large priming volumes, and so they required greater quantities of foreign blood. The major drawbacks, however, were the blood flow geometry, changing blood volumes, and blood mixing.
Membrane oxygenators are divided into two broad and overlapping categories. The first oxygenator type with flat membranes was discussed previously. The second oxygenator type is the capillary membrane oxygenator, in which the membranes are tubular rather than flat. The capillary membrane oxygenator offers several advantages: added strenght due to tubular arrangement and the capability of obtaining a large surface area in a relatively small space. The mode of operation of the capillary membrane oxygenators is the same as that of the flat membrane oxygenators; i.e., oxygen and carbon dioxide diffuse across the membranes at rates proportional to their respective partial pressure gradients.
Capillary membrane oxygenators are further divided into two classes: those with blood flowing through the tubes and oxygen flowing around the tubes, and those with oxygen flowing through the tubes and blood flowing around the tubes.
The vast majority of capillary membrane oxygenators are of the first type. Bodell et al (1963) were apparently the first to use capillary tubing to carry oxygen. They used small diameter silicone capillary tubing (inside diameter 0.012 inches and outside diameter 0.025 inches) of lengths reaching 100 feet. Bodell's model of a capillary membrane oxygenator consisted of a series of repeatable units containing several strands of the tubing wound in a helical loop. The loop formed a lumen through which the blood could flow. Several of these units were assembled in series to form the complete oxygenator. Oxygen transfer was adequate, and hemolysis rates were significantly lower than those of the bubble and disc oxygenators but were still high (66-172mg Hb/100 ml plasma after 90 minutes of continuous oxygenation). In addition, the unit was difficult to assemble, operate, and sterilize.